Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN_8

[Oleg Nesterov <oleg@redhat.com>]

On 04/26, Jacob Shin wrote:
>
> Implement hardware breakpoint address mask for AMD Family 16h and
> above processors. CPUID feature bit indicates hardware support for
> DRn_ADDR_MASK MSRs. These masks further qualify DRn/DR7 hardware
> breakpoint addresses to allow matching of larger addresses ranges.

Imho, looks good.

Just one nit and one question below.

> @@ -163,29 +165,8 @@ void arch_uninstall_hw_breakpoint(struct perf_event *bp)
>  	*dr7 &= ~__encode_dr7(i, info->len, info->type);
...
> +	if (info->mask)
> +		set_dr_addr_mask(0, i);

I agree we should clear addr_mask anyway.

But I am just curious, what if we do not? I mean what will the hardware
do if this breakpoint was already disabled but the mask wasn't cleared?

> @@ -314,11 +300,14 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
>  	if (ret)
>  		return ret;
>  
> -	ret = -EINVAL;
> -
>  	switch (info->len) {
>  	case X86_BREAKPOINT_LEN_1:
>  		align = 0;
> +		if (info->mask) {
> +			if (!cpu_has_bpext)
> +				return -EOPNOTSUPP;
> +			align = info->mask;

OK. But it seems we need a CONFIG_CPU_SUP_AMD-dependant helper for
this cpu_has_bpext check? (like we have the nop set_dr_addr_mask()
variant if !CONFIG_CPU_SUP_AMD).

Suppose that the kernel was compiled without CONFIG_CPU_SUP_AMD.
Then perf_event_open(attr => { .bp_len == 16 }) will succeed, but
this breakpoint won't actually work as expected?

Oleg.